In a typical metal coating process, a strip of metal to be coated, after preliminary treatment, passes downwardly into a bath of molten coating metal, around a sink roll submerged in the bath, upwardly past at least one deflector roll located just below the surface of the bath, through jet stripping apparatus, and to and about a turn-round roll located above the bath.
The turn-round roll is the first solid object to contact the coated strip, and it is necessary for the coating to have solidified before contact is made. Having regard to the speed of operation of modern plants, the turn-round roll is, therefore, a considerable distance above the bath, even though strip coolers may be provided, so as to ensure that the coating solidifies before it reaches the roll.
Traditionally, jet stripping apparatus has comprised an elongate nozzle on each side of the strip transversely of the strip and directing a horizontal, substantially planar, jet stream of gas against the vertical strip.
Because of its length, the unsupported strip between the bath and the turn-round roll tends to vibrate. The vibrations cause variations in the distances between the strip and the respective nozzles and this results in objectionable variations in the coating thickness.
To overcome that disability it has been proposed to replace each single nozzle on each side of the strip with a dual-nozzle assembly comprising upper and lower, parallel nozzles, spaced apart by a reaction body.
Such a dual-nozzle apparatus is described in the complete specification of Australian patent No. 581081.
When that apparatus is operating a static pressure is developed in the space between the reaction body and the strip, which space is referred to as the "stabilizing zone" hereinafter.
The magnitude of the pressure in the stabilizing zone, for constant gas supply pressures to the nozzles, depends markedly upon the distance between the strip and the reaction body. If the strip departs from a mid-position between the reaction bodies, the pressure in the then narrower stabilizing zone rises, and the pressure in the then wider stabilizing zone falls, so that a restoring force is generated tending to maintain the strip at the desired mid-point and opposing any vibrational movement of the strip.
Such prior known dual-nozzle jet stripping apparatus is very effective at preventing variations in coating thickness due to strip vibration, but it has been found that it induces undesirable surface roughness into the finished coating.